In multiantenna radio communication systems based on OFDM (Orthogonal Frequency Division Multiplexing) in recent years, for example, LTE-A (Long Term Evolution-Advanced) under design, several downlink multi-base-station coordinated techniques including non-coherent downlink joint transmission (non-coherent joint transmission) and coordinated beamforming require mutually orthogonal demodulation reference signals (DM-RS) among coordinated cells. FIG. 1 shows an illustrative system framework of non-coherent downlink joint transmission and coordinated beamforming. As shown in FIG. 1, in non-coherent downlink joint transmission, different data streams are transmitted from base stations of cell 1 and cell 2 to a terminal (UE) in a coordinated manner. The terminal (UE) receives different data streams from base stations of different cells on the same time and frequency resources. Although these data streams are superimposed on the time and frequency, since the data streams originate from different base stations, they can be said to have passed through different channels or have different spatial characteristics. The terminal needs to classify the data streams using different spatial characteristics of the data streams. The terminal can measure the spatial characteristics of the data streams using reference signals inserted beforehand in predetermined time and space resources. If reference signals of different data streams are also superimposed on each other on the time and frequency, the terminal cannot measure spatial characteristics of the data streams based on these reference signals. Therefore, the terminal (UE) requires reference signals orthogonal to each other between cells so that they are used to detect different data streams from cell 1 and cell 2. In coordinated beamforming, base stations of cell 1 and cell 2 form specific beams to their respective terminals UE_A and UE_B through precoding. However, terminal UE_A and UE_B also receive interference from the base station of the neighboring cell. Therefore, terminal UE_A and UE_B need reference signals orthogonal to each other between the cells to improve accuracy of channel estimation with respect to the cells.
In the LTE-A system under discussion, a base station of each cell transmits different pieces of data to terminals using resource blocks (RB) of the same time/frequency resources. FIG. 2 shows one resource block in the LTE-A system. A resource block is made up of resource elements (REs). The horizontal axis direction in FIG. 2 shows time and the vertical axis direction shows frequency, and one column in the vertical axis direction represents one OFDM symbol and DM-RSs are arranged beforehand in several resource elements in the resource block. The terminal performs interpolation using DM-RSs in the resource block, estimates channel conditions on other REs in the resource block and further performs demodulation. In the LTE-A, DM-RSs and data in the resource block are precoded for a specific terminal. In other words, precoding used for different terminals may not be the same. Even for the same terminal, channel conditions on different resource blocks differ from each other, and therefore precoding may not be identical. Since the terminal can only perform channel estimation using a resource block using the same precoding scheme, the terminal can actually only estimate a condition of a channel occupied by the resource block using DM-RSs inside the one resource block. That is, it is not possible to estimate a channel condition inside the resource block using DM-RSs of other than one resource block. In this case, to use an interpolation algorithm more effectively, it is necessary to arrange DM-RSs on an OFDM symbol (symbol) as close as possible to an edge of the resource block and improve the accuracy of interpolation. FIG. 2 illustrates the situation in which DM-RSs are arranged following this policy. As shown in FIG. 2, the first three columns of the resource block are a reserved control region and cannot be used for an arrangement of DM-RSs. For this reason, DM-RSs are arranged on two edge OFDM symbols in the data region (that is, portion other than the control region) of the resource block.
In view of the above-described situation, when arranging DM-RSs in a resource block, it is preferable to arrange DM-RSs at edges of the resource block as much as possible and at the same time maintain orthogonality of DM-RSs between the cells. One arrangement scheme that can be easily thought of in this regard is shown in FIG. 3. As shown in FIG. 3, the density of DM-RSs in a resource block of each cell is 12 DM-RSs per resource block. Six DM-RSs are arranged in each of two edge OFDM symbols of a data block for resource blocks of cell 1 and cell 2. In order for DM-RSs of cell 1 and cell 2 to be orthogonal to each other, an offset exists in DM-RSs of cell 2 with respect to DM-RSs of cell 1 on the frequency and data puncturing is performed at positions corresponding to the DM-RSs of cell 2 in the resource block of cell 1, and vice versa.